We have two principal objectives in this project. First, we will undertake an electron microscopic search for recombination complexes in vegetative and meiotic cells of diploid yeast (Saccharomyces cerevisiae). We expect to isolate chromosomal DNA from spheroplasts in various stages of meiosis and from spheroplasts from vegetative cells that have been treated with ultraviolet light and chemical mutagens to induce high frequencies of gene conversion and crossing over. Second, we will continue the investigation of the action of nonsense suppressors and of genes modifying the effects of suppression.